With the rapid development of information technology and burgeoning expansion in high-tech industry, an uninterruptible power supply (UPS) has been used as an emergent power supply device for a great amount of electronic devices. Most of sophisticated electric instruments and facilities count on high-quality power supply to maintain a normal operation condition. Currently the UPS has become an optimum solution to ensure the best power supply quality in the event of power outage.
Despite the advantages that an UPS can provide, however, when a legacy UPS is put to maintenance or repair, the power conduction passageway of the UPS will be changed by a bypass switch. At this moment, the load will continue to be supported by the unprotected utility power, leaving the load vulnerable to the power grid. In order to solve this problem, a parallel redundant UPS system is proposed such that the load is protected thoroughly and the reliability of the whole power supply system is improved.
FIG. 1 shows a prior art 1+1 parallel redundant UPS system. The parallel 1+1 redundant UPS system shown in FIG. 1 includes a first UPS module UPS-1 and a second UPS module UPS-2. The first UPS module UPS-1 and the second UPS module UPS-2 are configured to be arranged with the same internal circuitries, wherein both comprise a filter/circuit breaker 11,21, a rectifier 12,22 a control switch 13,23, an inverter 14,24, an output circuit breaker 15,25, a bypass circuit 16,26, a battery 17,27, and a controller 18,28, respectively. Both of the first UPS module UPS-1 and the second UPS module UPS-2 are connected to an inlet 100 for coupling an input AC power source and an outlet 101 of a distributed power network for coupling to a load. The operation of an individual UPS module shown in FIG. 1 is as follows. The filter/circuit breaker 11,21 receives an input AC power from an input AC power source through the inlet 100 and filters the input AC power into a filtered AC power. The rectifier 12,22 converts the filtered AC power into a DC power having a predetermined voltage level. The control switch 13,23 receives the DC power from the rectifier 12,22 and also receives a DC power from the battery 17,27. The controller 18,28 determines whether the DC power received from the rectifier 12,22 is within a predetermined tolerance. If the DC power received from the rectifier 12,22 is within a predetermined tolerance, and then the controller 18,28 controls the control switch 13,23 to provide the DC power from the rectifier 12,22 to the inverter 14,24. If the DC power received from the rectifier 12,22 is not within a predetermined tolerance, which may occur due to power outage or power surge problems, the controller 18,28 controls the control switch 13,23 to provide the DC power from the battery 17,27 to the inverter 14,24. The inverter 14,24 receives a DC power under the control of the controller 18,28 and converts the DC power into an AC power, and in turn regulates the AC power to predetermined specifications. The output circuit breaker 15,25 is used to provide electrical isolation between the load and the UPS modules. The bypass circuit 16,26 is connected between the inlet 100 and the outlet 101 of the UPS system. In case of a failure occurred in the interior of an UPS module, power supply is changed to the bypass circuit 16,26 such that the input AC power source is directly couple to the load. In some cases, the rectifier 12,22 may include a battery charger circuitry for providing electric power to charge the battery 17,27 under a normal condition, and the controller 18,28 may transmit the operation status information of the UPS module to a user, either locally using an indicator or display device, or remotely using by communicating with an external monitoring device.
In normal condition, one of the UPS module is taken as a primary UPS module for supplying a critical load power and the other one is taken as a redundant UPS module. In emergency condition, upon failure of the input AC power source (blackout or brownout), both the rectifier 11,21 will shut off and the inverter 14,24 continues to power the load using the battery 17, 27. When the input AC power from the input AC power source is restored prior to complete battery discharge, the rectifier 12,22 automatically start providing power to the inverters 14,24 and simultaneously charge the battery 17,27.
However, in the circuit configuration of FIG. 1, each UPS module includes an individual battery. When the inverter of any one of the UPS modules is malfunctioned, the battery associated therewith is not available to other UPS modules. That would results in a waste in energy usage and an inefficiency in spatial utilization. To negate such unfavorable factors, an UPS system using a common battery to be shared among UPS modules is addressed in order to accommodate the greatest benefit for the battery.
Referring to FIG. 2, a prior art parallel redundant UPS system with common battery operation is illustrated. As shown in FIG. 2, the circuit arrangement of the parallel redundant UPS system is analogous to that of FIG. 1, except that a common battery 30 is located between a first UPS module UPS-1 and a second UPS module UPS-2. The introduction of the common battery 30 substantially reduces the discharging rate of battery and lengthen the backup time. Moreover, it also increases the reliability of UPS systems. Because of the combination of parallel redundancy and common battery arrangement, the parallel redundant UPS system of FIG. 2 has the benefits of module redundant, that is, the rectifier 12 can feed the power for the inverter 24. Also the parallel redundant UPS system with common battery operation can increase the MTBF (mean time between failure) of the UPS module significantly.
However, the prior art UPS system suffers from several disadvantages because of the unbalance among the input currents of the internal UPS modules. Referring to FIG. 1, the first UPS module and the second UPS module are interconnected by a switch Q6. In normal condition, the switch Q6 is opened so that the individual battery will be charged by the rectifier/charger 12,22 respectively. At this moment the rectifiers/chargers 12,22 are not connected in parallel with a distribution power network, there will not induce the problem of unbalanced input currents for the UPS system. In emergent condition, the switch Q6 is closed and the input ends of the rectifier/charger 12,22 are connected together by the distribution power network, the battery 17,27 starts providing DC power to the inverter 14,24 so as to produce backup AC power for the load. Because the input AC power source fails to provide input AC power to the UPS module, there will not induce the problem of unbalanced input currents for the UPS system. However, as described above this configuration has the negative factors of inefficient usage of battery power and costly manufacturing budget on battery.
Referring to FIG. 2, in normal condition, the control switches 13,23 are all turned on and the common battery 30 is charged via one of the UPS module. Therefore, the rectifiers/chargers 12, 22 are not connected in parallel with a distributed power network. Because the input impedance of each UPS module is different with one another, the input currents of the UPS modules become unbalanced. The unbalanced input currents may cause severe problems to the rectifier 12,22, for example, the rectifier 12,22 may be overloaded and the semiconductor components used therein may have a shorter lifetime due to overheating.
In view of the foregoing problems, there is a tendency to develop an emergent power supply system provided with an input current balancing function among backup power supply modules associated therewith.